Chemical Equilibrium
Lesson Objectives
• Describe the nature of a reversible reaction.
• Define chemical equilibrium.
• Write chemical equilibrium expressions from
balanced chemical equations.
Lesson Vocabulary
• irreversible and reversible reaction
• reactants and products
• forward and reverse reaction
• rate of reaction
• chemical equilibrium
• molar concentration (molarity)
• equilibrium constant
• coefficient
Irreversible and reversible reactions
The combustion of methane is an irreversible reaction
CH4 + 2O2 → CO2 + 2H2O
reactants
products
The formation of hydrogen iodide is a reversible reaction
H2 + I2 ↔ HI
Forward reaction: H2 + I2 → HI
Reverse reaction: HI → H2 + I2
In a reversible reaction the conversion of reactants to
products and the conversion of products to reactants
occur simultaneously.
reversible reaction animation
Chemical equilibrium
When the rates of the forward and reverse reactions have become equal
to one another, the reaction has achieved a state of balance, called
Chemical equilibrium.
The conditions and properties of a system at equilibrium are
summarized below:
1. The system must be closed, meaning no substances can enter or
leave the system.
2. Equilibrium is a dynamic process. Even though we don’t observe
any changes, both the forward and reverse reactions are still taking
place.
3. The rates of the forward and reverse reactions must be equal.
4. The amounts of reactants and products do not have to be equal.
However, after equilibrium is attained, the amounts of reactants and
products (their concentration) will remain constant.
The Equilibrium Constant (Keq)
Consider the hypothetical reversible reaction in which reactants A and B
react to form products C and D:
aA + bB  cC + dD
(lower case letters, a,b,c,d represent the coefficients of each substance).
The rates of the forward and reverse reactions are the same at
equilibrium, and so the concentrations of all of the substances are
constant.
The equilibrium constant (Keq) is the ratio of the concentrations of
the products to the concentrations of the reactants, at
equilibrium. Each concentration is raised to the power of its
coefficient.
c
d
Keq = _______
[C] [D]
[A]a[B]b
The concentrations of each substance, indicated by the square brackets
around the formula, are measured in molarity units (mol/L).
The value of the equilibrium constant for any reaction does not
depend on the starting concentrations, so the equilibrium
constant has the same value regardless of the initial amounts of
each reaction component.
It does, however, depend on the temperature of the reaction.
Determining the Equilibrium Constant
The equilibrium expression only shows those substances
whose concentrations are variable during the reaction.
Therefore, an equilibrium expression omits pure solids
(s) and liquids (l) and only shows the concentrations of
gases (g) and aqueous solutions (aq).
1) Write the equilibrium expressions for the following reactions:
•
•
•
H2(g) + I2(g)  2HI(g)
2NH3(g) + 3 CuO(s)  3H2O(g) + N2(g) + 3Cu(s)
HCN(s) + H2O(l)  H+(aq) + CN-(aq)
2) Calculate the equilibrium expressions for the following reaction:
2NO(g) + O2(g)  2NO2
knowing that at 230°C, the equilibrium concentrations are
measured to be [NO] = 0.0542 M, [O2] = 0.127 M, and
[NO2] = 15.5 M.
Interpretation of the Equilibrium Constant
[Products]
K
[Reactants]
For K>1, we can state that the amount of
product is greater than the amount of
 reactant. The equilibrium is favored to
the right.
Likewise, for K<1, the amount of
reactants is much greater than the
products. The equilibrium favored to
the left.
Lesson Summary
• A reversible reaction is one in which products are converted to reactants
as well as reactants being converted to products. Equilibrium is achieved
when the rate of the forward reaction is equal to the rate of the reverse
reaction. Once at equilibrium, the concentrations of all substances remain
constant, and no net change occurs in the system.
• An equilibrium constant can be calculated for any reaction. Once a
reaction reaches equilibrium, the ratio of the mathematical product of all
product concentrations to the mathematical product of all reactant
concentrations, each raised to the power of its coefficient, will always be
equal to its equilibrium constant at that temperature. Note that solid and
liquid reaction components are not included in the calculation of the
equilibrium constant, since their concentrations are not dependent on the
amount present and thus do not change over the course of the reaction.
• An equilibrium constant greater than 1 indicates that the products of the
reaction as written are favored. An equilibrium constant less than 1
indicates that the reactants are favored.